Class, Lass and Ass
Tom was going to meet his students on the next day, so he wrote some
words on the blackboard which read as follows: "Professor Tom will
meet the class tomorrow."
A student, seeing his chance to display his sense of humor after reading
the notice, walked up and erased the "c" in the word "class." The
Professor noticing the laughter, wheeled around, walked back, looked
at the student, then at the notice with the "c" erased--calmly walked
up and erased the "l" in "lass", looked at the flabbergasted student
and proceeded on his way.